MIPS function continued

1. MIPS function continued

2. Stack • Key things to keep in mind: • Stack is a software concept – last in first out, that’s it. • In MIPS, you implement the stack by yourself by keeping $sp always pointing to the top element on the stack • Stack can be used in functions to save register values, and is the standard approach to save register values. But • You can also use stack for other purposes • This is not the only way to save register values.

3. .data msg: .asciiz "hello world" endl: .asciiz "\n" .text .globl main main: addi $sp,$sp,-1 sb $0,0($sp) la $t1, msg L0: lb $t0,0($t1) beq $t0,$0, L1 addi $sp,$sp,-1 sb $t0,0($sp) addi $t1,$t1,1 j L0 L1: la $t1,msg L2: lb $t0,0($sp) addi $sp,$sp,1 sb $t0,0($t1) beq $t0, $0, L3 addi $t1,$t1,1 j L2 L3: la $a0,msg li $v0,4 syscall la $a0,endl li $v0,4 syscall li $v0,10 #exit syscall

4. Implementing a Recursive Function • Suppose we want to implement this in MIPS: • It is a recursive function – a function that calls itself. • It will keep on calling itself, with different parameters, until a terminating condition is met.

5. The Recursive Function • What happens if we call fact(4)? • First time call fact, compare 4 with 1, no less than 1, call fact again – fact(3). • Second time call fact, compare 3 with 1, no less than 1, call fact again – fact(2). • Third time call fact, compare 2 with 1, no less than 1, call fact again – fact(1). • Fourth time call fact, compare 1 with 1, no less than 1, call fact again – fact(0). • Fifth time call fact, compare 0 with 1, less than 1, return 1. • Return to the time when fact(0) was called (when calling fact(1)). Multiply 1 with 1, return 1. • Return to the time when fact(1) was called (when calling fact(2)). Multiply 2 with 1, return 2. • Return to the time when fact(2) was called (when calling fact(3)). Multiply 3 with 2, return 6. • Return to the time when fact(3) was called (when calling fact(4)). Multiply 4 with 6, return 24.

6. The Recursive Function • In MIPS, we say calling a function as going to the function. So we go to the function over and over again, until the terminating condition is met. • Here, the function is called “fact,” so we will have a line of code inside the fact function: jal fact

7. The Recursive Function • The parameter should be passed in $a0. In the C function, every time we call fact, we call with n-1. So, in the MIPS function, before we do “jal fact”, we should have “addi $a0, $a0,-1.” addi $a0, $a0, -1 jal fact

8. The Recursive Function • After calling fact, we multiply the return result with n, so addi $a0, $a0, -1 jal fact mul $v0, $v0, $a0

9. The Recursive Function • After multiplying, we return, so addi $a0, $a0, -1 jal fact mul $v0, $v0, $a0 jr $ra

10. The Recursive Function • So, one if else branch is done. The other branch is slti $t0, $a0, 1 beq $t0, $0, L1 ori $v0, $0, 1 jr $ra where L1 is where we should go to if n >= 1.

11. The Recursive Function fact: slti $t0, $a0, 1 beq $t0, $zero, L1 ori $v0, $0, 1 jr $ra L1: addi $a0, $a0, -1 jal fact mul $v0, $v0, $a0 jr $ra Any problems?

12. The Recursive Function fact: addi $sp, $sp, -4 sw $ra, 0($sp) slti $t0, $a0, 1 beq $t0, $zero, L1 ori $v0, $0, 1 lw $ra, 0($sp) addi $sp, $sp, 4 jr $ra L1: addi $a0, $a0, -1 jal fact mul $v0, $v0, $a0 lw $ra, 0($sp) addi $sp, $sp, 4 jr $ra • The problem is that the function will call itself, as we have expected, but it will not return correctly! • We need to save $ra, because we made another function call inside the function. We should always do so. • Is this enough?

13. The Recursive Function • So now we can return to the main function, but the return result is 0, why? • A call to fact modifies $a0. But when we return from a call, we multiply it with $a0! • So, should also save $a0! • Restore it before using it again.

14. The Recursive Function week04-3.ppt

15. The Stack During Recursion week04-3.ppt

16. .data .text .globl main main: li $a0, 4 jal fact done: li $v0,10 syscall fact: addi $sp, $sp, -8 sw $ra, 4($sp) sw $a0, 0($sp) slti $t0, $a0, 1 beq $t0, $zero, L1 ori $v0, $0, 1 addi $sp, $sp, 8 jr $ra L1: addi $a0, $a0, -1 jal fact lw $ra, 4($sp) lw $a0, 0($sp) mul $v0, $v0, $a0 addi $sp, $sp, 8 jr $ra

17. Two other MIPS pointers • $fp: When you call a C function, the function may declare an array of size 100 like int A[100]. It is on the stack. You would want to access it, but the stack pointer may keep changing, so you need a fixed reference. $fp is the “frame pointer,” which should always point to the first word that is used by this function. • $gp: the “global pointer.” A reference to access the static data.

18. Solution to the in class exercise .data A: .word 0,1,2,3,4,5,6,7,8,9 .text .globl main main: la $a0,A li $a1,6 li $a2,5 jal onUpStream done: li $v0, 10 # exit syscall onUpStream: onUpStream_LOOP: lw $t0, 0($a0) beq $t0, $a1, onUpStream_NO beq $t0, $a2, onUpStream_YES addi $a0, $a0, 4 j onUpStream_LOOP onUpStream_YES: ori $v0, $0, 1 j onUpStream_DONE onUpStream_NO: ori $v0, $0, 0 onUpStream_DONE: jr $ra